ABSTRACT
The structure and dynamics of the solutions of LiPF6 in propylene carbonate over a concentration range of 0-3 mol/kg are studied with neutron spin echo spectroscopy, alternating-current (AC) conductometry, and shear impedance spectroscopy. The neutron diffraction shows a prepeak at ≈10 nm(-1) in addition to the main peak at ≈14 nm(-1) when the concentration of the salt is no less than 2 mol/kg. Compared with the frequency-dependent shear viscosity and AC conductivity, the relaxation of the shear stress agrees with that expected from the structural relaxation of the main peak. On the other hand, the relaxation of the conductivity is slower than the shear relaxation at all the concentrations, and the former approximately matches with the relaxation of the prepeak at the highest concentration, 3 mol/kg, which is several times slower than that of the main peak. The possible contribution of the prepeak structure to the ionic conduction is discussed.
ABSTRACT
The frequency-dependent viscosity and conductivity of three imidazolium-based ionic liquids were measured at several temperatures in the MHz region, and the results are compared with the intermediate scattering functions determined by neutron spin echo spectroscopy. The relaxations of both the conductivity and the viscosity agree with that of the intermediate scattering function at the ionic correlation when the relaxation time is short. As the relaxation time increases, the relaxations of the two transport properties deviate to lower frequencies than that of the ionic structure. The deviation begins at a shorter relaxation time for viscosity than for conductivity, which explains the fractional Walden rule between the zero-frequency values of the shear viscosity and the molar conductivity.